On public areas on the Net and conversations therein
There have been essentially public areas on the 'net for a long time (from the time before the 'net was the Internet). In all of that time, a pattern that repeats over and over is that they get used for what I'll call closed discussions among an in crowd. These discussions happen in public (in a Usenet newsgroup, on a public mailing list or website, on IRC, on Twitter, etc) so they're not private, but they're not public in the usual sense because they're not open to outside participants to butt in on. When this closed nature is not supported by the technology of the medium (which it usually isn't), it will instead be supported by social mores and practices, including ignoring people and the equivalent of mail filters and Usenet killfiles. There may be flaming or mockery of transgressors involved, too.
(If you want an analogy, what is going on is much like a group of people having a discussion at a restaurant table with you one table over. You can hear them fine and they are in 'public', but of course very few people think it's correct to turn around and join in their discussion and doing so rarely gets a good reaction from the group.)
What this means is that a conversation taking place in nominal public is not necessarily an open invitation for outside people to comment, and if they do they may be summarily ignored or find that there are bad reactions to their words. Equally, it's wrong to assert something like 'all conversations in public must include anyone who wants to participate' or the equivalent, because this is not how things work in practice in the real world (either on the 'net or off it).
As I mentioned, people on the 'net have been doing this with public spaces for a very long time now; this behavior is not at all novel or unusual. People who are shocked, shocked to see this happening in any particular instance (especially when they are shoving themselves into other people's discussions) are at best disingenuous. Wherever they are, people make groups and then talk among themselves.
There are also genuinely open public discussions in those public areas of the 'net, which creates obvious possibilities for confusion and misunderstandings. The cues for closed discussions are not always clear and some number of closed discussions are in practice only semi-closed; if you fit in, you can join in the conversation (indeed, this is how many such discussion groups expand). One way to assess the line between good faith misunderstandings of a situation and something else is the degree of stubborn persistence exhibited by the outsider.